Time compression/expansion systems are known in the prior art and in recent years versions employing repetitive variable time delay for pitch correction such as disclosed and claimed in U.S. Pat. No. 3,786,195 have become commercially available. In such systems, time compression refers to the playback of a recording at a speed higher than that at which the original work was recorded in order to complete the playback of the record in a shorter time than was taken for performance of the original work. Under such conditions, a normal record reproducer will produce an output audible signal which has its pitch frequencies transformed by the same factor as the time compression, i.e., the ratio of playback time to original recording time. In order to utilize time compression effectively where the factor is significantly greater than one such that a substantial saving in time is realized, pitch correction has been applied to the signal to return the pitch of the output signal to something approximately the normal pitch of the speaker or singer or other audible material originally recorded. Where approximately the original pitch is restored to the audible signal obtained with time compression it becomes possible to listen to recorded material at twice the speed of its original recording thus using only approximately one-half the time necessary to listen to a given speech, lecture or other recorded material and this speed listening is actually less tiring, if the pitch is properly corrected, than listening to the somewhat slow pace of delivery by the original speaker, for example. Time compression at greater than a factor of two is also possible with such systems although the greater the increase in speed more familiarity with such listening sensation is required for comprehension. Time expansion is actually the opposite of time compression where the record is played back at a slower speed resulting in a reduction of the pitch frequencies obtained from the record and requiring pitch correction which increases the frequencies of the signals obtained from the slowly played record to restore them to approximately normal frequency. Time expansion is useful where difficult material is being absorbed by the listener such as foreign language learning and other situations where comprehension of the text may be difficult or impaired by natural reasons. In the present invention, time compression/expansion is considered a generic term as involving compression or expansion or both within the capability of a given apparatus as disclosed or claimed.
As described in considerable detail in U.S. Pat. No. 3,786,195, when repetitive variable time delay is used for pitch correction special measures are usually required to compensate for the discontinuity which occurs when the variable delay repeats. Thus filtering, blanking and gap filling with other signal components are suggested for use either alone or in combination in that patent. More sophisticated techniques for processing the signal including the discontinuity are found in U.S. Pat. Nos. 3,828,361; 3,846,827; and 3,869,708. While the techniques shown in these prior art patents produce very high quality audio reproduction at altered speeds with pitch correction, the complete elimination of the switching transient and the filling of the gap left by clearing and refilling the delay medium in certain forms of delay devices can result in still audible vestiges of the interfering transient signal. For extremely high quality record reproduction such as that required for radio and television broadcasting, for example, it is essential that the audio noise components be reduced to the level where they are imperceptible to the untrained ear. This quality is particularly required where the program material includes commercial messages which are presented on commercial radio and television broadcast stations at extremely high cost to the sponsor of the commercial message who thus will accept nothing short of perfection in the end product. On the other hand, the broadcast station is faced with the problem of presenting a pre-recorded message precisely within the interval of a predetermined time slot which is built into the recorded program material. If the recorded commercial message can be expanded or compressed exactly to fit the predetermined time slot the production and programming chores for network and studio broadcasting would be greatly simplified.